This invention relates to the retinoid X receptor, referred to hereafter as xe2x80x9cRXR.xe2x80x9d More particularly, it relates to the identification of its ligands, and the ramifications of this identification.
The nuclear receptor (NR) superfamily comprises more than 150 different proteins, most of which are believed to function as ligand activated transcription factors, exerting widely different biological responses by regulating gene expression (for review, see Di Croce et al, EMBO J1 8:6201-6210 (1999); Mangelsdorf, et al Cell 83:825-839 (1995); Perlmann, et al, Cell 90:391-397 (1997)). Members of this family include receptors for endogenous small, lipophilic molecules, such as steroid hormones, retinoids, vitamin D and thyroid hormone. In addition, many members of this family lack known ligands and are therefore referred to as xe2x80x9corphan receptorsxe2x80x9d (for review, see Gigxc3xa8re, et al, Endocrine Rev 20:689-725 (1999); Kastner, et al, Cell 83:859-869 (1995)).
During recent years small, lipophilic ligands and activators have been identified for several orphan receptors, leading to new insights of profound impact. These findings have dramatically increased understanding of endocrinology and disease (Forman, et al Cell 81:687-693 (1997); Xu, et al, Mol. Cell 3:397-403 (1999); Makashima et al., Science 284:1362-1365 (1999); Parks et al., Science 284:1365-1368 (1999); Wang et al., Mol Cell 3:543-553 (1999); Janowski et al., Nature 383:728-731 (1996); Kliewer et al., Cell 92:73-82 (1998); Blumberg et al., Genes Dev. 12:31953205 (1998)); however, ligands and biological functions of most orphan receptors remain to be elucidated, emphasizing the importance of unraveling previously uncharacterized signaling pathways by identifying novel endogenous ligands for NRs (for review, see Mangelsdorf et al, Cell 83:841-850 (1995); Giguxc3xa8re, et al, supra.
The retinoid X receptor (RXR) is activated by the vitamin A metabolite 9-cis retinoic acid, which binds with high affinity to the RXR ligand binding domain. In addition, RXR has been shown to form heterodimers with a large number of NRs including the retinoic acid receptor (RAR) and several orphan receptors. RXRs are essential for development of the embryo as shown by gene ablation experiments. In addition, genetic analyses of mutant mice have indicated important functions in the adult, e.g. in the central nervous system.
Dietary fatty acids have long been recognized as essential for normal growth and development, as energy fuels, membrane components and as precursors of essential lipidmetabolites (Salem et al., In Health Effects of Polyunsaturated Fatty Acids in Seafoods (1986); Neuringer et al., Ann Rev Nutr 8:517-541 (1998); Horrocks et al, Pharmacological Res 40:211-225 (1999); Xiang and Zetterstrom, Acta Paediatr 88:78-82 (1999)). Fatty acids are classified as xe2x80x9cnon-essentialxe2x80x9d and xe2x80x9cessential,xe2x80x9d respectively. Essentially fatty acids cannot be synthesized in humans and must be taken up via the diet. These include linoleic acid (18:2nxe2x88x926), alpha-linolenic acid (18:3nxe2x88x923), and their elongation and desaturation products arachidonic acid (20:4nxe2x88x926) and docosahexaenoic acid (DHA; 22:6nxe2x88x923), respectively (for a description of fatty acid nomenclature, see e.g. Neuringer et al., supra. Arachidonic acid has been shown to be the major precursor of eicosanoids, a large family of biologically active factors including prostaglandins, prostacyclins, leukotrienes and hydroxy fatty acids. These metabolites act as chemical transmitters both within and between cells, and have been shown to regulate ion transport, hormone secretion and the immune response. Interestingly, several arachidonic acid metabolites and other fatty acids have been shown to function as ligands for peroxisomeproliferator activated receptors, demonstrating that they can function as mediators of NR signaling cascades, in vivo.
In contrast to arachidonic acid metabolites, the mechanism(s) of action of DHA is not well understood. Interestingly, DHA accumulates at high levels in the postnatal mammalian CNS indicating that DHA is involved in the maturation of the CNS and/or in neurological processes. Notably, DNA deficiencies lead to neurological abnormalities and diminished learning ability in man (see Gamoh, et al, Neurosci 93:237-241 (1999); Fernstrom, Lipids 34:161-169 (1999); Sheaff Greiner, et al, Lipids Suppl 34:239-243 (1999)). Moreover, dietary DHA may be beneficial in treatment of atherosclerosis, inflammation and cancer (Horrocks, et al, Pharmacol Res 40:211-225 (1999); Rose, et al, Pharmacol Theraput 83:217-244 (1999)).
It has now been ascertained that DHA functions as a ligand for RXRs. As is elucidated herein, DHA may exert its biological activities, in whole or in part, by way of activating RXRs. This pathway has not been described in the prior art, and is a feature of the invention, as will be elucidated herein.